1. Technical Field
The present invention relates to a speed reducer which reduces an input rotational speed to be output.
2. Related Art
Most of the power obtained from a power source such as a motor is too high in its rotational speed or is too insufficient to be used as it is. In this respect, a technique has been typically used in which the rotational speed is reduced to an appropriate rotational speed using a speed reducer to obtain a necessary rotation number and a necessary torque.
As a speed reducer which obtains a large reduction ratio, the following has been proposed. Firstly, a revolving gear which has a smaller size than a ring gear and has a smaller number of teeth (for example, by one tooth) than the ring gear is installed inside the ring gear. A circular cam is installed to be able to rotate with respect to the revolving gear in the central position of the revolving gear. A first rotational shaft is vertically installed in the circular cam in a position on the central axis of the ring gear. In the speed reducer having such a configuration, if the circular cam rotates around the central axis of the ring gear due to the first rotational shaft, the revolving gear revolves around the central axis of the ring gear while being engaged with the ring gear. Further, while the revolving gear revolves around the central axis of the ring gear by one turn, the revolving gear rotates in a direction opposite to its revolving direction by the teeth number difference with the ring gear. Thus, by extracting the rotating motion of the revolving gear, it is possible to considerably reduce the input rotational speed (rotational speed of the first rotational shaft).
Further, the rotating motion of the revolving gear is extracted by a through hole formed in the revolving gear and a penetration pin inserted in the through hole. A clearance is formed between the through hole and the penetration pin. The rotating motion of the revolving gear can be extracted by the penetration pin while the revolving motion of the revolving gear is being absorbed by the clearance. The rotating motion of the revolving gear extracted by the penetration pin in this way is output to the outside through a second rotational shaft to which the penetration pin is connected (JP-A-2008-240852).
However, in JP-A-2008-240852 as described above, there is a problem that a backlash easily occurs. That is, while the input of the first rotational shaft is output through the second rotational shaft, in addition to a normal backlash occurring in a portion where the ring gear and the revolving gear are engaged with each other, a backlash also occurs in a portion where the through hole and the penetration pin of the revolving gear are in contact with each other. The latter backlash occurs due to a manufacturing error in the speed reducer having an operation principle of JP-A-2008-240852. Further, since these two kinds of backlashes are present, a large backlash easily occurs as a whole. As a result, there is a problem that a period occurs during which an output torque is not obtained with respect to the input of the first rotational shaft, or a large rattling in the rotational direction is generated in the second rotational shaft. For this reason, the size of the backlash is inspected after assembly, and if there is a problem, the speed reducer is disassembled, a specific component is changed into a component having a slightly different dimension, the speed reducer is assembled again, and the backlash is then inspected. By repeating these processes, it is possible to achieve a speed reducer having a backlash of a predetermined size or less. Alternatively, the dimensions of all the components are measured and sorted in advance, and the speed reducer is assembled with the components combined so that the size of the backlash is within an allowable range in assembly, which results in a considerably long time for assembly.